My experience of conquest is that you lose all the army members as occupation forces, and that you can then request 1-3 of them (seemingly randomly selected, but obviously not the administrator) with a messenger, so I expect to get back just enough soldiers to act as captains of replacement squads. My squads have consisted of naturalized citizens, which mainly means human bards and scholars, trained, send, and lost.
Unfortunately, there's a horrible crash bug that corrupts the equipment menus and eventually crashes your fortress unless you disband all squads. My single attempt to bypass this by waiting two weeks and then recreate some squads resulted in an immediate crash on unpausing. I don't know if this bug is related to raiding/conquest, but I wouldn't be surprised if the corruption results when troops and/or squads return. As far as I know no research has been done along that line, but some rather significant investigation into crash saves has shown that most of them stop crashing if all squads are disbanded.
Thus, it might be possible to play a fortress in the old way, i.e. repel attacks, but not perform any attacks yourself, but, as mentioned, it has not been investigated if that is the case. If returning soldiers are the trigger corruption, it may or may not be returning squads and/or returning former squad members that causes the game to crash.
As far as I'm concerned, conquest is currently broken beyond repair (you may be able to play for several years before the game is corrupted, but none of my 0.44.X fortresses that were able to raid (dead civ raiding is completely broken: squads can be sent out, but they neither arrive nor return) survived for 20 years).